This invention relates to an optical radar system for sensing the distance to obstacles using a search beam, and more particularly, to an optical radar system having a variable sensing area and hence improved performance using a search beam which can swivel in accordance with the steering angle of a steering wheel.
One conventional radar system for vehicles is disclosed in Tokkaisho (Japanese unexamined published patent application) No. 55-86000 published on June 28, 1980. The radar system disclosed in this application is an electromagnetic wave radar system. However, recently, it has been suggested to use optical radar employing a laser search beam in vehicles.
This optical radar system includes a control unit which produces and processes electrical signals, an optical transmitter which radiates a search beam having a predetermined wavelength, and an optical receiver which collects light reflected by reflective objects and transduces the collected light into a corresponding electrical signal.
Furthermore, the control unit includes a pulse modulator which produces a trigger signal input to a signal processing unit, the signal being generated simultaneously with a drive pulse signal. Drive pulse signal causes a light-emitting element fixed to the optical transmitter to produce a corresponding light pulse. The light pulse is then focused by a lens into a beam which is transmitted in front of the vehicle. Weak light reflected by an object is focused by a large-diameter lens. The reflected light focused by the lens passes through an optical filter which filters out background noise (external lights such as solar light, artificial illumination, etc.), enters a light-receiving surface of a light-sensitive element positioned at the focal point of the lens, and is transduced into a corresponding electrical reflection signal consisting of narrow, low-amplitude pulses. The reflection signal is input to, amplified and shaped by a wide-band amplifier of the control unit into a high-amplitude pulse signal which is then output to the signal processing unit. The processing unit derives the propagation delay of reflection light pulse relative to the light pulse radiated by the optical transmitter from the time relationship between the trigger signal and the pulse signal, thereby allowing calculation of the distance to the object in a well-known manner.
In the above radar, the beam transmitted by the optical transmitter is designed so as to have a slight angle of divergence in order to ensure detection of objects in front of and relatively close to the vehicle.
When the angle of divergence and direction of the beam are fixed, the detection distance limit (beyond which objects are not regarded as obstacles even if sensed) is restricted so as not to detect objects in the adjacent lanes while the vehicle travels along a straight road. An object present outside of the detection distant limit in front of the vehicle cannot be detected.
On the other hand, when the vehicle is travelling along a curved road, the beam is greatly offset out of the vehicle lane, so that a preceding vehicle travelling along the curved lane cannot be tracked. The deviation of the beam from the curved lane increases as the radius of curvature of the road decreases.